The GOP hopeful has gotten much of his campaign money from big donors, but now his fundraisers are looking for new sources of cash.

Reporting from Washington — After locking up much of the Republican establishment's heavyweight fundraisers, Mitt Romney is going after the little guy.

"Donate $3 today to be automatically entered to be Mitt's special guest for Election Night on Super Tuesday," reads an email appeal from the Romney campaign to supporters. A dramatic new Web video asks for $20 contributions to fight the "Obama Attack Machine."

The GOP presidential candidate made a similar solicitation after clinching the Michigan primary Tuesday, with a rare personal plea for people to visit his website and "pledge your support — in every way possible."

The pitch reflects a challenge for Romney, one of the wealthiest candidates ever to run for president: He so far has been unable to build a broad base of small donors who can inject fresh cash into his campaign. The need to do so has grown more acute as he heads into next week's Super Tuesday, with contests in 10 states, and subsequent battles in the fight for the GOP nomination.

Though he has outstripped his Republican rivals in fundraising, he also is burning through cash. Romney spent money nearly three times faster than he raised it in January, leaving him with $7.7 million. Since then, his campaign has shelled out at least $2.7 million for television advertising alone, according to sources familiar with the ad buys.

Romney, who put more than $40 million of his own money into his 2008 White House bid, has not yet pulled out his checkbook this year. Campaign officials declined to say whether he was considering doing so.

So far, the former Massachusetts governor has relied on a network of wealthy contributors to finance his run. Just 9% of the nearly $63 million Romney raised through the end of January came from supporters who gave $200 or less, according to an analysis by the nonpartisan Campaign Finance Institute. In contrast, almost half the money given to his GOP rivals Rick Santorum and Newt Gingrich, as well as President Obama, came from small donors.

Romney's campaign is trying to reverse that imbalance by soliciting single-digit donations via the Web. That's a tactic regularly used by the Obama campaign to gather new email addresses for future fundraising.

But capturing the enthusiasm of small donors requires a candidate to ignite the base, and some Republican fundraisers say that's something Romney has so far failed to do.

"Money comes in when something revolutionary has happened, when lightning strikes," said Peter Pasi, a GOP digital fundraising consultant whose clients this year include Santorum. "You have to have a bottle to catch the lightning — but there needs to be lightning there in the first place."

Romney's wealth, which he has pegged at "between $150 and $200 some odd million," could work against him.

"When he says, 'I need three bucks,' does it seem real?" Pasi asked.

Other fundraisers said that while Romney's campaign had excelled at assembling a top-flight network of major donors, it had been slow to woo the kind of ideologically driven small givers who have fueled the candidacies of Rep. Ron Paul, Rep. Michele Bachmann and former Alaska Gov. Sarah Palin.

Some strategists not affiliated with the Romney campaign said that it should be spending more on conservative voter lists and online advertising.

Romney campaign officials rejected that criticism, saying their success at raising money in large sums has overshadowed the work they have done to reach small donors.

"We have some of the best minds in online fundraising in the country," said digital director Zac Moffat. He added that there had not been a need to boost spending on Web ads because supporters were coming to the campaign on their own.

"In the last 60 days, we've seen a massive increase in people's participation," he said.

Almost 106,000 people gave the campaign $250 or less through the end of January, he noted, and 88% of the campaign's donors have not yet given the $2,500 maximum.

But an analysis by the Campaign Finance Institute found that Romney relies more heavily on large donors than any other major presidential candidate in the last 12 years. Two-thirds of the money he has raised so far has come from donors who have given $2,500.

That's in part because soliciting money from big donors is more cost-efficient. With a few phone calls, well-connected donors can raise thousands of additional dollars, while getting the same amount from small givers requires substantial investments in direct mail, phone banking and email lists.

The emphasis on getting big checks "reflects a smart finance strategy on the part of the campaign team, as well as a cost-effective approach and is the right way to build your initial support," said veteran GOP fundraiser Fred Malek, who is backing Romney.

But after his wins this week in Arizona and Michigan, GOP strategists said the former governor was well-positioned to jump-start a more ambitious small-donor program.

"A lot of small dollars flow with momentum, and I think he's got a lot of opportunity now with two wins," said Guy Short, a senior advisor to Bachmann who oversaw her enormous success among small donors. "Certainly, there is a large part of the Republican base that he has not successfully closed the sale with. He needs to make an argument why he can defeat Barack Obama and why conservatives ought to be comfortable with him."

Ultimately, if Romney continues to outpace his rivals, small donors will gravitate to him, said Nancy Bocskor, a GOP fundraiser and professor of fundraising at George Washington University's Graduate School of Political Management.